Add Benchmarking to parse and merge calls of Document.

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In #61, the Document shared across the FFI went from a *mut Document to a Arc<Mutex<Document>>. There's bound to be runtime overhead and we need to have a way to track the benchmarking of:
Document::parse and Document::parse_fragment_json.

It might be worth doing from Swift or Kotlin via the uniffi bindings but a good start is with cargo bench via criterion or something.

Device specific benchmarking would be cool but sonos/dinghy#81 might be a blocker.

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Closed in #104.